Bsi bs en 50647 2017

BS EN 50647:2017 BSI Standards Publication Basic standard for the evaluation of workers' exposure to electric and magnetic fields from equipment and installations for the production, transmission and distribution of electricity BS EN 50647:2017 BRITISH STANDARD National foreword This British Standard is the UK implementation of EN 50647:2017 The UK participation in its preparation was entrusted to Technical Committee GEL/106, Human exposure to low frequency and high frequency electromagnetic radiation A list of organizations represented on this committee can be obtained on request to its secretary This publication does not purport to include all the necessary provisions of a contract Users are responsible for its correct application © The British Standards Institution 2017 Published by BSI Standards Limited 2017 ISBN 978 580 93202 ICS 17.220.20; 17.240 Compliance with a British Standard cannot confer immunity from legal obligations This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 July 2017 Amendments/corrigenda issued since publication Date Text affected EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM EN 50647 BS EN 50647:2017 June 201 ICS 7.220.20; 7.240 English Version Basic standard for the evaluation of workers' exposure to electric and magnetic fields from equipment and installations for the production, transmission and distribution of electricity Norme fondamentale pour l'évaluation de l'exposition des travailleurs aux champs électriques et magnétiques produits par les équipements et installations de production, transport et distribution d'électricité Basisnorm für die Evaluierung der beruflichen Exposition gegenüber elektrischen und magnetischen Feldern ausgehend von Komponenten und Anlagen zur Erzeugung, Übertragung und Verteilung elektrischer Energie This European Standard was approved by CENELEC on 201 7-04-1 CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member This European Standard exists in three official versions (English, French, German) A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung CEN-CENELEC Management Centre: Avenue Marnix 7, B-1 000 Brussels © 201 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members Ref No EN 50647:201 E BS EN 50647:2017 EN 50647:201 (E) Contents Page European foreword 3.1 Terms and definitions 3.2 Physical quantities and units 3.3 Abbreviations 6.1 General 1 6.2 Exposure assessment regarding external fields 1 6.2.1 General 1 6.2.2 Harmonics of magnetic field 6.2.3 Harmonics of electric field 6.3 Numerical calculation of induced electric fields inside the human body 8.1 General 8.2 Simplified criteria for compliance with action levels 8.2.1 General 8.2.2 Magnetic fields 8.2.3 Electric fields 8.3 Assessment using measurements or calculations 8.3.1 General 8.3.2 Magnetic fields 8.3.3 Electric fields 9.1 General 9.2 Simplified criteria for compliance with exposure limit values 9.2.1 General 9.2.2 Magnetic fields 20 9.2.3 Electric fields 21 9.3 Assessment using dosimetry and considerations for non-uniform fields 21 3.1 Workers at particular risk 23 3.2 Other requirements 23 Annex A (informative) Assessment of harmonics in magnetic fields 24 A.1 Introduction 24 A.2 Assessment Method using TEI 24 A.3 Assessment using the weighted peak function 26 A.4 Simplified assessment procedure for public grids 28 Annex B (normative) 50 Hz magnetic field sources in the environment of equipment and installations for production, transmission and distribution of electricity 29 B.1 General 29 B.2 Currents in single conductors 29 B.3 Currents in circuits 31 B.4 Assessing magnetic fields exposures 31 B.5 Check list for assessing compliance for magnetic fields 33 Annex C (informative) Examples of application of the different assessment criteria 34 BS EN 50647:2017 EN 50647:201 (E) Assessment for air-cored reactors: Simplified calculation of the magnetic field under a vertical air-cored self-inductance 34 C.2 Assessment for insulated cables: Calculation of compliance distances for typical XLPE cables 36 C.3 Assessment for exposure to electric fields considering different coupling conditions 38 Annex D (informative) Method for deriving Exposure-Limit-Equivalent-Fields (LEFs) 41 D.1 Introduction 41 D.2 Method 41 D.3 Selection of the reference model: 42 D.4 Reference organs and data 42 D.5 Uncertainty assessment 43 D.6 Deriving the Exposure-Limit-Equivalent-Field (LEF) 44 Annex E (informative) Considerations about DC magnetic fields in electrical companies 45 E.1 Introduction 45 E.2 Exposure of workers to DC magnetic field in electrical companies 45 E.3 Attention points 45 Annex F (informative) contact currents 46 F.1 Introduction 46 F.2 Influence of electric fields 46 F.2.1 General 46 F.2.2 Person isolated (at floating potential), capacitive coupling to ground 46 F.2.3 Person at earth potential, isolated object 47 F.2.4 Spark discharges 48 F.3 Influence of magnetic fields 48 F.3.1 General 48 F.3.2 Working adjacent to live circuits 48 F.4 Summary 49 Annex G (informative) Exposure during transient and fault conditions 50 G.1 Introduction 50 G.2 Faults 50 G.2.1 Overview 50 G.2.2 Short-circuit currents during faults 50 G.2.3 Prevention and protection against faults 50 G.2.4 Magnetic field exposures during faults 51 G.3 Switching transients 51 G.4 Lightning strikes 51 G.5 Inrush currents 51 G.6 Compliance of short-duration events with the Directive 52 Bibliography 53 C.1 BS EN 50647:2017 EN 50647:201 (E) European foreword This document [EN 50647: 201 7] has been prepared by CLC/TC 06X “Electromagnetic fields in the human environment” The following dates are fixed: • • latest date by which this document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 201 8-04-1 latest date by which the national standards conflicting with this document have to be withdrawn (dow) 2020-04-1 Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights CENELEC shall not be held responsible for identifying any or all such patent rights This document has been prepared under a mandate given to CENELEC by the European Commission and the European Free Trade Association BS EN 50647:2017 EN 50647:201 (E) Scope This European Standard provides a general procedure to assess workers’ exposure to electric and magnetic fields (EMF) in work places associated with the production, transmission and distribution of electric energy, and to demonstrate compliance with exposure limit values and action levels as stated in the Council and European Parliament “EMF” Directive 201 3/35/EU [1 ] NOTE The Council and European Parliament Directive 201 3/35/EU will be transposed into national legislation in all the EU member countries It is important that users of this standard consult the national legislation related to this transposition in order to identify the national regulations and requirements These national regulations and requirements may have additional requirements that are not covered by this standard It has the role of a specific workplace standard It takes into account the non-binding application guide for implementing the EMF Directive [1 0] and it defines the assessment procedures and compliance criteria applicable to the electric industry The frequency range of this standard covers from DC to 20 kHz, which is sufficient to include the power frequency used for electric power supply systems throughout Europe (50 Hz) and the various harmonics and inter-harmonics occurring in the supply system In this extremely low frequency range, electric and magnetic fields are independent and, therefore, they both have to be addressed in the exposure assessment NOTE Electrical companies also use radio frequency transmissions to operate and maintain their networks and power plants Similarly, other exposures to EMF may occur during maintenance operations, for instance, due to the use of hand-held electrical tools All these EMF sources are outside the scope of this standard NOTE Regarding EMF in the low frequency range, the scientific basis of the EMF directive is the ICNIRP health guidelines published in 201 [1 3] Reference is made to this scientific basis when necessary for justifying or clarifying some of the technical statements of the present document Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application For dated references, only the edition cited applies For undated references, the latest edition of the referenced document (including any amendments) applies EN 61 786-1 , Measurement of DC magnetic, AC magnetic and AC electric fields from Hz to 100 kHz with regard to exposure of human beings - Part 1: Requirements for measuring instruments (IEC 61786-1) EN 50527-1 , Procedure for the assessment of the exposure to electromagnetic fields of workers bearing active implantable medical devices - Part 1: General EN 50527-2-1 , Procedure for the assessment of the exposure to electromagnetic fields of workers bearing active implantable medical devices - Part 2-1: Specific assessment for workers with cardiac pacemakers IEC 61 786-2, Measurement of DC magnetic, AC magnetic and AC electric fields from Hz to 100 kHz with regard to exposure of human beings - Part 2: Basic standard for measurements BS EN 50647:2017 EN 50647:201 (E) Terms, definitions, physical quantities, units and abbreviations 3.1 Terms and definitions For the purposes of this document, the following terms and definitions apply 3.1 action level AL operational level established for the purpose of simplifying the process of demonstrating compliance with the relevant exposure limit value or, where appropriate, to take relevant protection or prevention measures Note to entry: “Reference levels” as defined in the European Recommendation 999/51 9/EC [3] for limiting the exposure of the public and in ICNIRP Health Guidelines [1 3] are based on the same approach as Action Levels and the two terms are defined to achieve the same objective Note to entry: For electric fields, “Low ALs” and “High ALs” are levels which relate to the specific protection or prevention measures specified in the EMF Directive [1 ] Note to entry: The Low AL for external electric field is based both on limiting the internal electric field below ELVs and on limiting spark discharges in the working environment Below the High AL, the internal electric field does not exceed ELVs and annoying spark discharges are prevented, provided that the relevant protection measures are taken Note to entry: For magnetic fields, “Low ALs” are levels which relate to the sensory effects ELVs and “High ALs” to the health effects ELVs Note to entry: Compliance with the ALs will ensure compliance with the relevant ELVs If the assessed exposure values are higher than the ALs, it does not necessarily follow that the ELVs have been exceeded, but a more detailed analysis is necessary to demonstrate compliance with the ELVs Note to entry: ALs may not provide adequate protection to workers at particular risks, for whom a particular risk assessment shall be performed 3.1 compliance distance distance from a source of field that ensures respect of the relevant exposure limit values or action levels Note to entry: Working at distances smaller than compliance distances requires a specific assessment 3.1 contact current current between a person in established contact with a conductive object, resulting from the inductive or capacitive coupling between the field and the person and/or object, and expressed in amperes (A) Note to entry: The EMF directive [1 ] specifies limits for the steady-state value of the contact current 3.1 electric field constituent of an electromagnetic field which is characterized by the electric field strength E together with the electric flux density D Note to entry: In French, the term “champ électrique ” is also used for the quantity electric field strength [SOURCE: IEV, ref 21 -1 -67] 3.1 exposure index EI assessed exposure divided by the relevant action level or exposure limit value BS EN 50647:2017 EN 50647:201 (E) 3.1 exposure-limit-equivalent field LEF magnitude of uniform external electric or magnetic field that exposes the person to the sensory or health effects ELV Note to entry: The numerical values of LEFs are derived from dosimetry 3.1 exposure limit value ELV limit which is based directly on established health effects and biological considerations Note to entry: In the frequency range covered by the present standard, ELVs are expressed in terms of induced electric fields except between Hz and Hz where the ELV is given in terms of external magnetic field Note to entry: “Basic restrictions” as defined in the European Recommendation 999/51 9/EC [3] for limiting the exposure of the public and in ICNIRP Health Guidelines [1 3] are based on the same approach as Exposure Limit Values and the two terms are defined to achieve the same objective 3.1 exposure limit values for sensory effects (sensory effects ELVs) ELVs above which workers might be subject to transiently disturbed sensory perceptions, i.e retinal phosphenes and minor changes in brain functions Note to entry: The sensory effects relate only to the central nervous system of the head Exceeding sensory effects ELVs is allowed under controlled conditions for informed workers 3.1 exposure limit values for health effects (health effects ELVs) ELVs above which workers might be might be subject to adverse health effects, such as stimulation of nerve and muscle tissue Note to entry: Compliance with health effects ELVs will ensure that workers exposed to electric and magnetic fields are protected against all established adverse health effects Note to entry: The threshold for muscle excitation is far higher than for nerve excitation, and therefore the directive [1 ], consistently with its scientific basis [1 3], considers limits to prevent only nerve excitation, as they are conservative with regard to muscle excitation As a result, the health effects relate to the peripheral nervous system, i.e the whole body 3.1 induced electric field electric field inside a human body resulting directly from an exposure to an external source of electric or magnetic field 3.1 1 magnetic field constituent of an electromagnetic field which is characterized by the magnetic field strength the magnetic flux density B H together with Note to entry: In French, the term “champ magnétique” is also used for the quantity magnetic field strength Note to entry: In this document, the term magnetic field is used for magnetic flux density [SOURCE: IEV ref 21 -1 -69] BS EN 50647:2017 EN 50647:201 (E) 3.1 total exposure index TEI sum of all exposure indexes (e.g at different frequencies, or from different sources) of either electric or magnetic field Note to entry: If the total exposure index is less than one, the exposure is compliant Note to entry: Using the arithmetic sum makes the TEI a conservative assessment of exposure 3.1 unperturbed field field at a point that would exist in the absence of persons or in the absence of movable objects which are not necessary for the work progress Note to entry: All limits expressed in terms of fields external to the human body refer to the unperturbed field [SOURCE: EN 61 786-1 :201 4, 3.3.1 ] 3.2 Physical quantities and units For the purposes of this document, the physical quantities and units given in Table apply Table — Physical quantities and units Quantity Electric field strength Magnetic flux density Contact current Symbol E B Ic 3.3 Abbreviations AL CNS DC EI ELV GIS HV LEF PNS TEI WPR rms action level central nervous system direct current exposure index exposure limit value gas insulated substation high voltage exposure limit equivalent field peripheral nervous system total exposure index worker at particular risk root mean square Unit V/m T A Comments µT is more usually used at 50 Hz BS EN 50647:2017 EN 50647:201 (E) D.3 Selection of the reference model: The different models considered in the review are given in Table D.1 (the names of the models are explained in [1 5]) The closest model to the ICRP reference man is Maxwell Only models with results relevant for comparison with ELVs, i.e expressed in terms of induced electric field, are considered here Table D.1 — Comparison of representative male human body models mass (g) adult ICRP Adipose tissues Blood Bone, total Brain Heart tissue only Heart with blood Kidneys Liver Lung with blood Marrow, bone Muscle, skeletal Skin Total height (in m) Total mass (in kg) 200 600 500 450 330 840 31 800 200 650 29 000 300 ,76 73 male UVIC (calculated from volume) 029 031 484 377 440 38 055 532 ,77 76 NORMAN MAXWEL DUKE 221 737 039 118 471 340 1 800 532 355 900 370 332 877 029 348 952 044 750 360 240 80 30 427 06 ,76 73 28 602 31 ,76 73 34 00 500 ,74 70 D.4 Reference organs and data The sensory effects ELV aims at protecting the CNS tissue of the head Relevant organs are thus the brain (separated into white and grey matter in some body models), the retina and the optical nerve The spinal cord is mostly not in the head, so is therefore not included in considerations of the sensory effects ELV The health effects ELV aims at protecting the CNS and PNS of the whole body IC NIRP 201 specifies that the skin should be taken into account: “ There is no conversion factor for peripheral nerve tissue available at present Therefore, the skin, which contains peripheral nerve endings, was chosen as a worst-case target tissue.” Hence, the following organs were considered for the PNS ELVs: the brain, the retina, the optical nerve, the spinal cord and the skin It should be noted that taking all organs into account would raise a problem of coherence between models because the most critical organ depends on the model or because the results are not available for all organs Table D.2 gives the induced electric field in different organs of the body and for different field orientations of the magnetic field For the electric field, only the vertical orientation for a human body connected to the ground is presented as it is the worst case exposure situation In order to avoid numerical singularities, the criteria used to express the maximum induced field is the 99th percentile of the distribution in each organ, as recommended by ICNIRP In the Table D.2, the highest value is highlighted, also defining the most critical organ with regard to sensory effect (in practice: the brain) and health effects (in practice: the skin) 42 BS EN 50647:2017 EN 50647:201 (E) Table D.2 — Induced electric field (mV/m; 99th criterion) for the reference model (Maxwel) mT LAT * mT AP ** mT TOP*** kV/m TOP*** 7,2 34,5 33,9 7,5 7,7 53,3 33,9 27,9 42,2 48,2 25,6 26,3 27,2 4,3 8,7 00,0 53,8 43,8 78,9 29,7 3,8 26,3 9,1 3,7 6,9 52,9 43,2 22,7 41 ,9 23,0 5,7 ,9 ,0 ,0 0,6 4,4 2,4 7,3 4,8 3,5 blood brain grey matter brain white matter csf eye retina fat heart muscle skin spinal cord *LAT: horizontal field, lateral exposure (i.e side to side) **AP: horizontal field with an orientation antero-posterior (i.e face to back) ***TOP: vertical field (i.e top to bottom) D.5 Uncertainty assessment Various parameters potentially influencing the computation results have been considered (the detailed justification is given in [1 5]): — computation method; — meshing of the human body; — conductivity of tissues; — post-processing; — morphology of the model The calculated uncertainty for the brain exposed to a magnetic field is 37 % The detailed calculation is given in Table D.3: Table D.3 — Calculation uncertainty for magnetic field regarding central nervous system Uncertainty sources Morphology (for the brain) Conductivity Calculation method Resolution size Post processing Combined standard uncertainty Value of the uncertainty Division factor Probability distribution uvi ki Sensitivity coefficient c i Standard uncertainty ui = uvi / ki ±2,5·1 − Rectangular √3 1 ,44·1 − ±3,0·1 − negligible ±5,0·1 − ±5,0·1 − Rectangular Rectangular Rectangular Rectangular √3 1 1 ,73·1 − 288·1 − 2,88·1 − m u c = ∑ c i u √3 √3 √3 i 3,67·1 − 43 BS EN 50647:2017 EN 50647:201 (E) A similar calculation results in a combined uncertainty of 38 % for the skin which is the relevant organ for considering effects on the peripheral nervous system For exposure to electric fields the calculated uncertainty is 50 % for the brain, and 44 % for the skin D.6 Deriving the Exposure-Limit-Equivalent-Field (LEF) Regarding exposure to magnetic fields, the highest induced electric field is 34,5 mV/m in the brain for a mT LAT exposure (see Table D.2) The corresponding calculated uncertainty is 37 % (formally 36,7 % see Table D.3) Adding this uncertainty to the calculated values finally results in a conservative estimate of the highest induced field in the brain: 47, mV/m for a mT uniform field exposure The 50 Hz magnetic field equivalent to the exposure limit for sensory effects (1 00 mV/m in the central nervous system) is therefore 2,1 mT, conservatively rounded to mT Following the same approach, the highest induced field in the skin is 09 mV/m The resulting LEF for health effects (800 mV/m in the peripheral nervous system) is thus 7, 34 mT, conservatively rounded to mT For electric fields both the two effects, on central and peripheral nervous systems, result in the same rounded value: 35 kV/m Table D.4 — Exposure-limit-equivalent-fields LEFs (lowest value of uniform field corresponding to exposure limit values) Lowest value of uniform field corresponding to the exposure limit values Sensory effects 44 Health effects 50 Hz magnetic field mT mT 50 Hz electric field 35 kV/m 35 kV/m BS EN 50647:2017 EN 50647:201 (E) Annex E (informative) Considerations about DC magnetic fields in electrical companies E.1 Introduction Directive 201 3/35/EU defines two ALs for exposure to static magnetic field: — For the hazard of interference with Active Implanted Medical Devices (AIMD): 0,5 mT; — For the hazard of attraction and projection of metallic objects in the fringe field close to high field (>1 00 mT) sources: mT This is the case close to MRI installations, where the magnetic field inside the coil is more than T The ELVs for static magnetic field are still much higher: T (normal working conditions) and T (localized limbs exposure) E.2 Exposure of workers to DC magnetic field in electrical companies Most of electric current transmission and distribution is AC at 50 Hz in Europe, but some power lines are DC and their number is growing (for example with subsea cables) Such lines generate a static magnetic field Maximum exposure situations are found in the close vicinity of lines and cables, and inside AC/DC conversion substations When analysing exposure to DC magnetic field, it should be considered that there exists a background exposure due to the Earth’s magnetic field (between 30 µT and 70 µT depending on the proximity to the magnetic poles) With regard to exposure of workers close to cables, the formulas given in Annex B are applicable They show than in any case, the ELVs are never exceeded A number of particular sources have been found in electricity generation, such as: — Generator excitation: the DC magnetic field measured close to the exciter of a turbogenerator (eg inside the cabinet) has been found to be of order of mT — Overhead conveyor belt for the transportation of coal: the static magnetic field can be created by a permanent magnet or by an electromagnet for separation of ferromagnetic impurities The measured static magnetic field is in the order of 00 mT just under the magnet and in the order of mT at 20 cm Such power magnets are sometimes signalled with warning « forbidden for cardiac implant » E.3 Attention points The exposure to static magnetic field is always lower than the ELVs, but some situations have been identified where the exposure can, very locally, be higher than the ALs: — close to generator exciters; — close to overhead conveyor belt magnets; — close to HVDC cables 45 BS EN 50647:2017 EN 50647:201 (E) Annex F (informative) contact currents F.1 Introduction Contact currents are indirect effects of electric and magnetic fields that occur when a person comes in contact with a conductive object (usually a metallic structure) when the person and/or the object are under the influence of the field Different cases have to be considered depending on the type of field (electric or magnetic) and the isolation level of the person and/or the object with respect to the earth The cases considered hereunder assume that the minimum electrical safety clearances as specified by IEC / CEN [6] [7] are complied with The safety rules adopted by electrical companies are designed to ensure that these minimum clearances are maintained F.2 Influence of electric fields F.2.1 General An electric field induces, by capacitive coupling, a voltage on people and objects that are isolated from earth, i.e at floating potential When contact is made between a person and an object, a current appears that tries to cancel the difference of potential Depending on the respective level of insulation of person and object, different cases have to be considered F.2.2 Person isolated (at floating potential), capacitive coupling to ground This exposure situation can be simply modelled (Figure F.1 ) In practice the isolation is never perfect and a leakage current to ground always reduces the floating potential of the person Nevertheless under dry conditions, this leakage can be neglected Figure F.1 — Capacitive coupling for an isolated person exposed to an electric field The capacitive current flowing in a person standing in a vertical uniform field is about µA per kV/m [1 ; 0] If the person comes into contact with an earthed object, the contact current will be of the same order of magnitude 46 BS EN 50647:2017 EN 50647:201 (E) Figure F.2 — Contact with a grounded structure and equivalent electric circuit The contact current Ic is independent from the contact impedance Zc as long as this impedance is negligible compared to the capacitance of the body to the ground C o , i.e Zc < < (2π f C ) −1 In practice, the capacitance C o is about (1 50-200) pF, and the practical condition for neglecting the contact impedance is Zc < M Ω, wh ich wil l al ways be the case for establ ish ed contacts Work positions above ground in substations have been investigated [4] The contact currents (hand to grounded structure) were measured and the exposure to electric fields was assessed using different sets of measurements (legs, trunk, head, and averaged) A consistent correlation was observed with a ratio of µA per kV/m of electric field averaged over the body Under this correlation, the mA limit for contact current would be reached for an averaged exposure of 55 kV/m, what never happens in practice Hence the contact current between a person and a grounded structure will always be much lower than mA For a worker climbing on a tower the field can become highly non uniform Nevertheless, for most work positions the coupling between the field and body is reduced (see C.3) and for a given level of exposure to electric fields, the contact current is lower than in the reference exposure situation, i.e a man standing at ground level exposed to a vertical electric field F.2.3 Person at earth potential, isolated object According to several studies [1 0], [1 6], [1 ] the contact current between a grounded person and a vehicle isolated from ground ranges from 0,05 mA per kV/m (small car) up to 0,5 mA per kV/m (truck, bus… ) This means a contact current up to mA in a field of kV/m NOTE The situation of a grounded worker is somewhat theoretical as security shoes are normally not conducting The use of conducting shoes is usual for live line working and also for workers climbing towers with energized circuits Figure F.3 — Contact current for a grounded person touching an isolated vehicle 47 BS EN 50647:2017 EN 50647:201 (E) For an isolated conductor in the vicinity of the earth (e.g a fence), the magnitude of the contact current is worth about 0,2 to 0,3 mA per kV/m for 00 m of influence This means about mA for a 00 m long conductor parallel to a power line This contact current is even higher for an isolated conductor in the vicinity of the power line (e.g a deenergised circuit in a double circuit line) Hence, the only way to ensure that the contact current remains lower than mA is to earth it in order to bring it at the same potential as the person The quality of the earthing is not critical (e.g a single rod is sufficient) Care should also be taken in relation to the induction effect of the magnetic field, which might require additional precautions (see F.3) If the person wears isolated gloves, the risk of contact currents is highly reduced If the person simply wears shoes with isolated soles, a capacitive current can still flow This current is normally smaller than mA F.2.4 Spark discharges Whatever insulation level of the person or object with respect to the earth, a transient discharge (capacitive spark discharge, also called “microshock”) usually occurs before the contact is established Such a spark discharge has orders of magnitude higher peak amplitude than the steady-state current that follows but is very short in duration (from less than µs to about 00 µs) Although not harmful, this kind of transient current is readily perceivable and can sometimes be experienced as unpleasant [1 7] [2] It can also, to some extent, be compared to the well-known electrostatic discharge that has higher peak amplitude but shorter time duration The Directive does not give quantitative limits on spark discharges but recommends that excessive spark discharges are prevented As no safety threshold is given, workers in electrical transmission companies should be taught to manage this unpleasant phenomenon: it is well known that the annoyance can be reduced if the contact is established on a large surface (e.g the palm of the hand) rather than a small one (e.g the top of a finger) The possible pain can also be reduced if the grasping contact is established promptly or if the first contact is made through a metallic item (for example a tool) that the worker grasps with their hand, or if it is made by contacting a less sensitive part of the body (for example, the forearm is much less sensitive than the fingers) Using an equipotential bonding is also a possible solution to avoid spark discharges F.3 Influence of magnetic fields F.3.1 General As for electric fields, magnetic fields can induce voltages in conductive objects However, contrary to what happens with the capacitive coupling due to the electric field, the magnetic coupling occurs only when a significant parallelism exists between an object (e.g a conductor or a pipe) and the source of magnetic field (e.g an overhead line) Contrary also to the capacitive coupling, earthing the conductive object at a large distance from the point of contact will actually increase the amplitude of the contact current A parallelism of about 00 m can result in contact currents exceeding mA in normal operating conditions F.3.2 Working adjacent to live circuits A situation where significant currents can be induced by magnetic fields is when a circuit is earthed down so that work can be done on it, and where another circuit runs parallel with the first circuit and is carrying currents Under this situation, electrical companies apply appropriate safety procedures and rules to prevent the associated risk of electrical shock In any event, for safety reasons in case of an earth fault occurring on the power line, it is mandatory either to avoid the electrical contact by wearing gloves and isolated shoes or to ensure that the object is correctly earthed in the immediate vicinity of the point of contact (e.g less than m) 48 BS EN 50647:2017 EN 50647:201 (E) F.4 Summary Whether due to an electric or a magnetic field, contact currents can always be kept under the level of mA if the object with which the contact is made is earthed For the influence of an electric field, the place and the quality of the earthing are not relevant For the influence of a magnetic field, the earthing has to be done in the immediate vicinity (a tens of meters) of the point of contact and needs to be of good quality 49 BS EN 50647:2017 EN 50647:201 (E) Annex G (informative) Exposure during transient and fault conditions G.1 Introduction Normal operation of transmission and distribution systems involves various ways in which voltages or currents higher than normal are produced, but lasting only very short periods, typically only a few cycles of the 50 Hz waveform Examples are faults; switching transients; lightning strikes; and inrush currents This annex first gives further factual information about these events and the exposures they produce, then considers how the Directive applies to them G.2 Faults G.2.1 Overview Faults are unwanted events that occur on the electricity system from time to time They are rare because of the many preventative measures that are taken, but they are inherent in the operation of an electricity system and cannot be avoided altogether Faults have a variety of causes, including plant failure at power stations or on the transmission system, and weather-related events Lightning is the most common cause All faults occurring on an electrical transmission system are recorded and their causes are investigated as appropriate An order of magnitude is that there are approximately faults per year for 000 km of transmission system G.2.2 Short-circuit currents during faults The main characteristic of a fault is a short-duration increase in the current in one or more of the phases The voltage can also be affected but to a much lesser extent The current that flows depends on the circumstances surrounding the fault The highest current at any given point on the system is expressed as the “fault level”, which varies throughout the system and depends on the system impedances between the location of the fault and the sources of the power The highest fault level found on present-day transmission systems is typically around 60 kA (taking into account all the possible network system voltages), corresponding to the rating of the switchgear, which has to be able to break the highest fault current G.2.3 Prevention and protection against faults Faults are undesirable from an operational point of view because they disrupt the electricity supply and can cause damage to connected equipment Considerable efforts are devoted towards preventing faults from occurring Maintenance and replacement strategies, and the provision of earth wires on overhead lines, are designed to prevent them occurring in the first place, and protection systems are in place throughout the network to ensure faults are detected and isolated as soon as is technologically feasible after they have occurred The target fault clearance time (for the main in-feeding circuit) is about 00 ms for systems at 66 kV to 400 kV, i.e at most a few cycles of a 50 Hz wave Slightly longer values may apply when the fault is also being fed from a remote end, but in that instance, the fault current is also lower Once the fault has been cleared, the circuit can be switched back on, and the current, and the resulting exposures, will revert to normal levels On the low-voltage distribution systems where fuses are used, low-level faults may be sustained for longer than a second, but the current and therefore exposures would be correspondingly lower as well 50 BS EN 50647:2017 EN 50647:201 (E) G.2.4 Magnetic field exposures during faults In practice, faults not in fact normally result in exposures exceeding the ELV The highest fault level anywhere on a high-voltage transmission system is around 60 kA Fault levels on lower-voltage systems tend to be lower than this The highest magnetic-field exposure to someone standing on the ground close to an overhead line will result if a fault current flows in the bottom conductor The lowest permissible ground clearance for 400 kV circuits varies from country to country and with other factors such as the use of the land being crossed, but is typically no less than m In practice the ground clearance is usually greater than this At m above the ground, i.e m from a minimum-height conductor carrying 60 kA, the field would be 700 µT, less than the High Action Level In other words this worst-case fault situation would still be compliant with the Directive Furthermore, normally the line will be at a greater height, the fault may not be on the bottom phase, the current may be lower than the fault level, and the probability of someone being in that location when the fault occurs (usually during a thunder storm) is minimal The possibility of problems would therefore arise only for situations where workers are closer to the conductor than on the ground beneath an overhead line, such as when working on transmission towers, in substations, in cable tunnels or in the vicinity of underground three-phase cables The probability of a worker being in one of these locations at the instant of a fault is low In some circumstances, it is permissible to operate a circuit permanently under fault conditions, but this occurs only when the fault current is within the rating of the circuit, and therefore does not give rise to any higher exposures than normal operations G.3 Switching transients When switching the voltage on to an overhead-line circuit, the complex electrical parameters of the overhead line give rise to transient voltages For transmission circuits, these comprise a waveform at a higher frequency, typically of order kHz, superimposed on the 50 Hz waveform, and lasting typically no more than one cycle of the 50 Hz, 20 ms The initial amplitude of the higher frequency can be comparable to the amplitude of the 50 Hz, meaning that the peak voltage can be roughly doubled If, at the instant it is reenergised, the line still has charge on it from a previous energisation, higher peak voltages, up to typically three times the steady-state peak voltage, can be produced This transient voltage would give rise to a transient electric-field exposure to a worker located on a tower at the relevant place at the instant the voltage was applied G.4 Lightning strikes As already stated, lightning strikes are one of the more common causes of faults on transmission systems, and therefore are the indirect cause of the high currents that flow during a fault However, the lightning strike also increases the voltage of the circuit for the short duration it lasts for The maximum voltage is determined by the insulation properties of the circuit (when the voltage produced by the lightning strike exceeds the insulation withstand voltage, the voltage flashes over, which is the cause of the fault) This limits the peak voltage to, typically, three or four times the peak voltage under steady-state conditions The duration of this transient voltage is usually taken as less than ms G.5 Inrush currents Some loads that are connected to distribution systems, such as motors, produce an inrush current when first energised This can be larger than the steady-state current by a factor up to 32 but lasts typically half a cycle, ms 51 BS EN 50647:2017 EN 50647:201 (E) G.6 Compliance of short-duration events with the Directive In all these cases, the duration of any high exposures is strictly limited In the case of faults, this is ensured by the protection systems that disconnect a faulted circuit In the case of switching transients and inrush currents, it is ensured by the intrinsic characteristics of electrical circuits This therefore fulfils one of the requirements of Article (8): that immediate action should be taken to remove exposure in excess of the limits The probability of over-exposure actually occurring is extremely small, because it necessitates a worker to be present in a specific location (on the body of a tower with live circuits level with the conductors; close to the path of the fault current) at the instant that an already rare event (switching or fault) occurs If, however, this extremely low-probability exposure event does take place, the duration of the exposure is at most a few cycles of the 50 Hz waveform Such short exposures are unlikely to have significant biological effects In the absence of pragmatic guidance from ICNIRP, it is legitimate to draw on the corresponding IEEE C95.6 standard [1 4] which states that the averaging time for assessing exposures, based on time constant of nerve stimulation, should be 200 ms The temporary exposures discussed here are of less duration that this Faults that are of longer duration are generally of lower level, e.g some faults on lowvoltage distribution systems or, on transmission systems, faults that take longer to clear because they are remote from the protection Therefore, these transient exposures are extremely unlikely to constitute over-exposure, and no further action need be taken Where controlled fault currents are applied intentionally, for example for testing or for fault location, then planned protective measures may nonetheless need to be taken to ensure exposures remain lower than the ELVs 52 BS EN 50647:2017 EN 50647:201 (E) Bibliography [1 ] C IGRE W.G 36.01 Electric and magnetic fields produced by transmission systems 980, no Cigré Technical Brochure No 21 [2] CIGRE WG C4.25, Issues related to spark discharges [3] COUNCIL OF THE EUROPEAN UNION Council Recommendation of July 999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz) (1 999/51 9/EC) Official Journal of the European Communities 999, 99 (L) pp 59–70 [4] D ESCHAMPS F et al 201 Exposure to Electric Field at Work Positions High Above Ground Level in High Voltage Substations In Proceedings of the CIGRE ELF-EMF colloquium, Paris201 , paper C05 [5] Voltage characteristics of electricity supplied by public electricity networks EN 50341 , Overhead electrical lines exceeding AC 45 kV - Part 1: General requirements - Common specifications EN 62226-2-1 , Exposure to electric or magnetic fields in the low and intermediate frequency range Methods for calculating the current density and internal electric field induced in the human body - Part 2-1: Exposure to magnetic fields - 2D models (IEC 62226-2-1) EN 6231 , Assessment of electronic and electrical equipment related to human exposure restrictions for electromagnetic fields (0 Hz - 300 GHz) EPRI Transmission Line Reference Book – 345 kV and Above 982, no 01 974 EUROPEAN COMMISSION DIRECTORATE-GENERAL FOR EMPLOYMENT, S.A.A.I.U.B Nonbinding guide to good practice for implementing Directive 2013/35/EU Electromagnétic Fields Volume 1: Practical Guide Luxembourg: Publications Office of the European Union, 201 ISBN 978- [6] [7] [8] [9] [1 0] 2016 (under preparation) EN 501 60, 92-79-45869-9 [1 ] EUROPEAN PARLIAMENT AND COUNCIL OF THE EUROPEAN UNION Directive 201 3/35/EU of the European Parliament and of the Council on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (20th individual Directive within the meaning of Article 6(1 ) of Directive 89/391 /EEC) and repealing Directive 2004/40/EC Official Journal of the European Union 201 3, 79 (L) p 21 [1 2] F RIEDL K et al 201 Harmonic Factor Evaluation For Electric and Magnetic Fields Using Symmetrical Components In Proceedings of the CIRED, Frankfurt201 [1 3] ICNIRP Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 00 kHz) Health Phys 201 0, 99 (6) pp 81 8–836 [1 4] [1 5] IEEE STD C95.6, fields 2002 Standard for safety levels with respect to human exposure to electromagnetic M AGNE I., D ESCHAMPS F Electric field induced in the human body by uniform 50 Hz electric or magnetic fields: bibliography analysis and method for conservatively deriving measurable limits J Radiol Prot 201 6, 36 (3) pp 41 9–436 53 BS EN 50647:2017 EN 50647:201 (E) [1 6] [1 7] [1 8] 54 REILLY J.P Electric Field Induction of Long Objects - a Methodology for Transmission Line Impact Studies Power Apparatus and Systems IEEE Transactions on 979, PAS-98 (6) pp 841 –1 852 REILLY J.P., L ARKIN W.D Human Sensitivity to Electric Shock Induced by Power-Frequency Electric Fields Electromagnetic Compatibility IEEE Transactions on 987, EMC-29 (3) pp 221 –232 WORLD HEALTH ORGANIZATION Extremely Low Frequency (ELF) Fields, 984 This page deliberately left blank NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW British Standards Institution (BSI) BSI is the national 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